I. Field of the Invention
The present invention relates to communication systems which employ spread spectrum signals. More specifically, the present invention relates to a method and apparatus for processing orthogonal signals in a spread spectrum communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known in the art. However the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, the disclosure of which is incorporated by reference.
In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users each having a transceiver communicate through satellite repeaters or terrestrial base stations (also referred to as cell-sites stations, cell-sites or for short, cells) using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity.
The CDMA modulation techniques disclosed in U.S. Pat. No. 4,901,307 offer many advantages over narrow band modulation techniques used in communication systems employing satellite or terrestrial channels. The terrestrial channel poses special problems to any communication system particularly with respect to multipath signals. The use of CDMA techniques permits the special problems of the terrestrial channel to be overcome by mitigating the adverse effect of multipath, e.g. fading, while also exploiting the advantages thereof.
The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307 contemplates the use of coherent modulation and demodulation for both directions of the link in mobile-satellite communications. Accordingly, disclosed therein is the use of a pilot carrier signal as a coherent phase reference for the satellite-to-mobile link and the cell-to-mobile link. In the terrestrial cellular environment, however, the severity of multipath fading with the resulting phase disruption of the channel, as well as the power required to transmit a pilot carrier signal from the mobile, precludes usage of coherent demodulation techniques for the mobile-to-cell link. U.S. Pat. No. 5,103,459 entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", issued Jun. 25, 1990, assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference, provides a means for overcoming the adverse effects of multipath in the mobile-to-cell link by using noncoherent modulation and demodulation techniques.
In a CDMA cellular telephone system, the same frequency band can be used for communication in all cells. The CDMA waveform properties that provide processing gain are also used to discriminate between signals that occupy the same frequency band. Furthermore, the high speed pseudonoise (PN) modulation allows many different propagation paths to be separated, provided the difference in path delays exceed the PN chip duration, i.e. 1/bandwidth. If a PN chip rate of approximately 1 MHz is employed in a CDMA system, the full spread spectrum processing gain, equal to the ratio of the spread bandwidth to the system data rate, can be employed against paths that differ by more than one microsecond in path delay from the desired path. A one microsecond path delay differential corresponds to a differential path distance of approximately 1,000 feet. The urban environment typically provides differential path delays in excess of one microsecond, and up to 10-20 microseconds are reported in some areas. Multipath fading is not totally separated by using CDMA discrimination techniques because there will occasionally exist paths with delay differentials of less than the PN chip duration for the particular system. Signals having path delays on this order cannot be distinguished in the demodulator, resulting in some degree of fading.
Diversity is one approach for mitigating the deleterious effects of fading. It is, therefore, desirable that some form of diversity be provided which permits a system to reduce fading. Three major types of diversity exist: time diversity, frequency diversity and space diversity.
Time diversity can best be obtained by the use of repetition, time interleaving, and error correction and detection coding which like repetition introduces redundancy. A system comprising the present invention may employ each of these techniques as a form of time diversity.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile unit through two or more cell-sites usually by employing two or more antenna elements. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM", issued Mar. 21, 1992 and U.S. Pat. No. 5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", issued Apr. 28, 1992, both assigned to the assignee of the present invention.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109 entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", issued Oct. 8, 1991, also assigned to the assignee of the present invention.
The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307 entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", issued Feb. 13, 1990 and assigned to the assignee of the present invention, contemplate the use of relatively long PN sequences with each user channel being assigned a different PN sequence. The cross-correlation between different PN sequences and the autocorrelation of a PN sequence, for all time shifts other than zero, both have a nearly zero average value which allows the different user signals to be discriminated upon reception. (Autocorrelation and cross-correlation requires logical "0" take on a value of "1" and logical "1" take on a value of "-1" or a similar mapping in order to obtain a zero average value.)
However, such PN signals are not orthogonal. Although the cross-correlations essentially average to zero, for a short time interval such as an information bit time the cross-correlation is a random variable with a binomial distribution. As such, the signals interfere with each other much the same as if they were wide bandwidth Gaussian noise at the same power spectral density. Thus the other user signals, or mutual interference noise, ultimately limits the achievable system user capacity.
It is well known in the art that a set of n orthogonal binary sequences, each of length n, for n any power of 2 can be constructed, see Digital Communications with Space Applications, S. W. Golomb et al., Prentice-Hall, Inc., 1964, pp. 45-64. In fact, orthogonal binary sequence sets are also known for most lengths which are multiples of four and less than two hundred. One class of such sequences that is easy to generate is called the Walsh function, also known as Hadamard matrices.
A Walsh function of order n can be defined recursively as follows: ##EQU1## where W' denotes the logical complement of W, and W(1)=.vertline.0.vertline.. Thus, ##EQU2## A Walsh sequence or code is one of the rows of a Walsh function matrix. A Walsh function matrix of order n contains n sequences, each of length n bits.
A Walsh function metric of order n (as well as other orthogonal functions) has the property that over the interval of n code symbols, the cross-correlation between all the different sequences within the set is zero, provided that the sequences are time aligned with each other. This can be seen by noting that every sequence differs from every other sequence in exactly half of its bits. It should also be noted that there is always one sequence containing all zeroes and that all the other sequences contain half ones and half zeroes.
Walsh codes can be used to provide orthogonality between the users so that mutual interference is reduced, allowing higher capacity and better link performance. With orthogonal codes, the cross-correlation is zero over a predetermined time interval, resulting in no interference between the orthogonal codes, provided only that the code time frames are time aligned with each other.
To obtain the benefit of the orthogonal Walsh codes, the transmitter of a system may map code symbols into corresponding Walsh codes. For example a three bit symbol could be mapped into the eight sequences of W(8) given above. An "unmapping" of the Walsh encoded signals into an estimate of the original code symbols must be accomplished by the receiver of the system. A preferred "unmapping" or selection process produces soft decision data which can be provided to a decoder for maximum likelihood decoding.
A correlation receiver is used to perform the "unmapping" process. In such a receiver a correlation of the received signal with each of the possible mapping values is performed. Selection circuitry is employed to select the most likely correlation value, which is scaled and provided as soft decision data.
A spread spectrum receiver of the diversity or "Rake" receiver design comprises multiple data receivers to mitigate the effects of fading. Typically each data receiver is assigned to demodulate a different path propagation of the signal. In the demodulation of signals modulated according to an orthogonal signaling scheme, each data receiver correlates the received signal with each of the possible mapping values. Selection circuitry within each data receiver then selects the most likely correlation value. The values selected from all of the data receivers are scaled and combined to produce soft decision data.
In the just described process the selection circuitry injects a nonlinearity into the decoding process which may result in inaccurate soft decision data. Furthermore, the standard selection circuit can require a plurality of functions which may require substantial circuitry and thus increase the complexity, size, power consumption, and cost of a system, particularly when repeated in each data receiver.
It is therefore desirable to provide in a spread spectrum receiver of the type just described an enhanced decision process which eliminates nonlinearities associated with such selection circuitry. Since the selection circuitry is employed in each data receiver, it is further desirable to combine the functions performed by the selection circuitry into a single processing element to avoid the associated disadvantages of such redundant circuitry.
The present invention is thus an improved alternative method and apparatus for accurately converting orthogonally encoded data signals into soft decision data using a set of simple functions. The benefit of the present invention is increased when it is incorporated into a system employing multiple data receivers.